Lubricant return schemes for use in refrigerant cycle

ABSTRACT

A control algorithm is developed which takes corrective action in the event that system conditions indicate there may be an inadequate flow of lubricant in the system. In particular, if a discharge pressure is below a predetermined amount or if the suction modulation valve is throttled, there is a possibility of inadequate lubricant flow. The system control then turns off the condenser fan, and if that first step is not sufficient, may also turn on an evaporator heater and then control a suction modulation valve. A fail safe control loop also takes effect if the condition sensor appears to have failed.

RELATED APPLICATION

This application is a continuation of U.S. application Ser. No.10/876,052, which was filed Jun. 24, 2005.

BACKGROUND OF THE INVENTION

This invention relates to schemes for improving lubricant flow, and inparticular, the return of lubricant from various locations in arefrigerant cycle back to a compressor, to prevent an inadequate supplyof lubricant to the compressor.

Refrigerant cycles typically include a compressor for compressing arefrigerant and delivering that refrigerant downstream to a condenser.From the condenser the refrigerant passes to an expansion device thatexpands the refrigerant. From the expansion device, refrigerant moves toan evaporator. The evaporator is a heat exchanger that typically changesthe temperature in an environment to be conditioned. From theevaporator, the refrigerant may run through a modulation valve beforereturning to the compressor. The evaporator often captures refrigerantand lubricant as it has traveled through the refrigerant cycle.Lubricant is typically necessary in the compressor to lubricate themoving parts. However, as the lubricant is delivered to the compressor,it is often entrained in and with the refrigerant, and travels throughthe refrigerant cycle. While the evaporator is specifically mentioned,oil can accumulate in other locations, and the present invention assistsin returning oil from these other locations.

The conditions in the evaporator often result in lubricant leaving therefrigerant, and remaining trapped in the evaporator. At some point,there may be an inadequate supply of lubricant in the compressor, whichis undesirable. Oil retention in the evaporator can also adverselyaffect the heat transfer within the evaporator, reducing efficiency andcapacity of the unit, which is also undesirable.

SUMMARY OF THE INVENTION

In a disclosed embodiment of this invention, a number of schemes areidentified which are made operational once a control senses conditionsthat could lead to an inadequate supply of lubricant. Preferably, thecondition sensed is a discharge refrigerant pressure. Of course, otherconditions such as suction pressure, etc. can be sensed. One main schemeis to periodically turn off the fan driving air over the condenser. Thiswill increase the refrigerant pressure. As the refrigerant pressureincreases, the likelihood of lubricant remaining in the evaporatordecreases.

A control monitors whether this stoppage of the condenser fan iseffective to change conditions that are likely indicative of anapproaching lubricant shortage. If the conditions continue for a periodof time, then a second scheme is implemented. In one embodiment thesecond scheme includes turning on heater elements that are associatedwith the evaporator. This increases the load on the evaporator, andensures that refrigerant mass flow through the evaporator increases.

Increasing mass flow would tend to entrain more lubricant, and take thatlubricant back to the compressor. Further, the increased mass flowtraveling through the evaporator ensures that the temperature change inthe refrigerant is less at the evaporator. This reduces the likelihoodof the refrigerant being boiled out of a lubricant/refrigerant mixture,such that more lubricant is returned to the compressor.

In a third scheme, if the first two schemes do not operate to correctthe problem, a suction modulation valve (SMV) is actuated to open andallow increased mass flow of refrigerant to the compressor. Again, thisensures that lubricant is less likely to remain in the evaporator.

Of course, the disclosed sequences can be changed and the correctivesteps taken in any other order.

In another aspect of this invention, a method is devised to ensure thelubricant is returned to the compressor, even if the transducer forsupplying the discharge pressure information to the control is broken.The various schemes as mentioned above may be utilized under certainconditions if the transducer is no longer providing a valid signal.

The various features and advantages of this invention will becomeapparent to those skilled in the art from the following detaileddescription of the currently preferred embodiment. The drawings thataccompany the detailed description can be briefly described as follows.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates a standard refrigerant cycle.

FIG. 2 is a flowchart for operating the FIG. 1 refrigerant cycle.

FIG. 3 is a flow chart of a second aspect of this invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

A refrigerant cycle 20 is illustrated in FIG. 1 having a compressor 22compressing a refrigerant and delivering the refrigerant to a downstreamcondenser 24. A fan 25 blows air over the condenser 24. Typically, thecondenser is an outdoor heat exchanger.

From the condenser 24 refrigerant travels to an expansion device 26, andfrom the expansion device 26 to an evaporator 28. The evaporator is anindoor heat exchanger. As shown, a fan 30 blows air over the evaporator,and that air is typically delivered into an environment to beconditioned. As also shown schematically, heater elements 132 areassociated with the evaporator. These heater elements have been placedinto some prior art evaporators to melt ice that may accumulate on theevaporator.

A suction modulation valve 34 is positioned downstream of theevaporator, and upstream of the compressor 22. The suction modulationvalve is able to modulate the suction pressure of the refrigerantreaching the compressor 22, and is also a known component.

A control 35 is associated with the cycle 20, and operates the fan 25,heater coil 132 and the suction modulation valve 34. Further, as shown,a discharge pressure transducer (DPT) 23 is positioned downstream of thecompressor 22. This transducer provides a signal indicative of therefrigerant discharge pressure leaving the compressor.

As shown, an economizer heat exchanger 32 is incorporated into therefrigerant cycle 20. A tap line and economizer expansion device 33expands a portion of the refrigerant that is returned through a line 40to an intermediate point in the compression cycle of the compressor 22.As is known, a main flow of refrigerant passing downstream to the mainexpansion device 26 is subcooled by this economizer cycle. As alsoshown, an unloader line 31 selectively communicates refrigerant that hasbeen compressed in the compressor 22 back to a suction line for thecompressor. In a preferred embodiment, this unloader line can beincorporated with the refrigerant return line 40. More complex and lesscomplex refrigerant systems would benefit from the teachings of thisapplication.

As mentioned above, under certain conditions, the flow of lubricant oilthrough a heat exchanger (normally an evaporator heat exchanger wouldretain most of the oil, but some retention of oil can also be expectedin the condenser heat exchanger as well as economizer heat exchanger, ifit is present, and adjacent piping associated with any of the systemheat exchangers) can become inadequate, with too much oil retained inthe system. This can cause an inadequate supply of oil being returned tothe compressor 22 and/or detrimentally affect the heat exchanger coilheat transfer thus reducing the operating efficiency and capacity of therefrigerant cycle. Thus, a flowchart is shown in FIG. 2 of a controlalgorithm for at least periodically ensuring that lubricant is returnedto the compressor.

As shown in FIG. 2, once a cooling cycle has begun, a delay is left toallow the system to adjust to steady state. In the embodiment shown inFIG. 2, fifteen seconds delay is set. However, as with all numericalinformation provided on the flowcharts of FIGS. 2 and 3, this is merelyan example. Other time periods would be within the scope of thisinvention, as would other numerical values to any of the values shown inFIG. 2 or 3.

Once the delay has passed, the control passes to an IF box which askswhether DPT is less than a particular pressure. If the DPT is less thanthis predetermined pressure, and if the condenser fan 25 has been on fora certain period of time, then a determination is made that there is apotential likelihood of inadequate lubricant being returned to thecompressor 22.

If either the condenser fan 25 has not been on for a certain period oftime, or if the DPT is above the predetermined pressure (i.e., a no toeither), then the system simply returns to steady state monitoring.

However, if the answer to both questions is yes, the algorithm continueson to the step of turning off the condenser fan 25. The control thengoes to another IF box which asks periodically whether the DPT hasincreased above another value, shown here to be significantly higherthan the predetermined value at the initial IF box. As long as thepressure has not increased back to this higher number, the controlmaintains a loop. Once the DPT has increased up to this number, thecondenser fan is turned back on. By turning off the condenser fan, thedischarge pressure should be increased. This increased pressure ensureslubricant is driven from the evaporator by overcoming the frictionalresistance between the lubricant itself and between the lubricant andevaporator material. Also provided are the benefits of increasedsolubility (more refrigerant is driven into the lubricant) and reducedviscosity (refrigerant/lubricant mixture is “thinned” out).

As further shown in FIG. 2, the answer at the second IF box is alsoprovided back to other control loops. If the condenser fan has remainedoff for a period of time, then the control moves to a second controlscheme B having an IF box asking whether DPT is below another pressure(preferably much lower than either of the pressures asked in the controlscheme A), and whether the condenser fan has been off for a certainperiod of time. If the answer to either question is no, the controlmaintains a loop. On the other hand, if the answer to both is yes, theheaters 32 at the evaporator are turned on. This provides the benefit ofincreasing the heat load that must be absorbed by the evaporator 28 andthus increasing mass flow of refrigerant through the evaporator. Thiswill ensure lubricant is returned to the compressor. In addition, byincreasing the mass flow, the temperature change per unit of refrigerantin the evaporator is reduced. By reducing the amount of temperaturechange, one reduces the likelihood of boiling off refrigerant from arefrigerant/lubricant mixture, thus ensuring more of the lubricant isreturned to the compressor.

After the heaters are turned on, the system moves to asking whether theDPT has increased to a predetermined level. As long as the DPT has notyet reached that level, the loop will continue. Once the DPT has reachedthat level, the heaters 32 will be turned back off and the system willbe turned to the initial IF box.

At the same time, a method under this invention may include yet a thirdcontrol scheme C. If the DPT is less than a predetermined value, and theheaters have been on for a period of time, and the condenser fan hasbeen off for a period of time, then the suction modulation valve isopened to a relatively large opening for a predetermined period of time.This will ensure the amount of refrigerant passing to the compressorincreases, and also increases the amount of refrigerant leaving theevaporator. Increasing refrigerant flow through the evaporator againincreases the refrigerant/lubricant ratio thus increasing solubility anddecreasing viscosity. With the refrigerant/lubricant mixture “thinned”(viscosity lowered) and an increase in the free flow area, as a resultof the increased SMV opening, the lubricant is purged from theevaporator back to the compressor sump.

The present invention thus provides a method of monitoring operation ofthe refrigerant cycle 20, and moving to take steps that are likely toincrease the flow of lubricant back to the compressor in the eventconditions are indicative of the possibility of inadequate lubricantbeing returned.

It should be understood that other sequences of operation would comewithin the scope of this invention. That is, the three schemes A, B, andC can be utilized in any order, with the scheme B or C used initially,with any of the other two used second and then third. Of course, foreach of the selected sequence, it may not be necessary to use all threeschemes, as the problem of oil return may be solved by engaging only oneor two schemes.

FIG. 3 shows another flowchart for providing lubricant to the compressorin the event that the DPT sensor 23 has failed. As long as the DPTsensor is seen as valid, normal operation of the FIG. 2 flowchart willoccur. There are, of course, many known ways of monitoring sensorfeedback to determine whether a signal is valid. This invention does notextend to how the determination is made. Further, this aspect of theinvention would extend to other monitored conditions.

Should the signal appear to be invalid, the control asks whether theambient temperature is between two preset amounts. If the answer is yes,then the SMV is moved to a predetermined open amount for a particularnumber of minutes per period of time. The condenser fan is maintainedon.

If the ambient temperature is not between that first range, the controlasks whether the ambient temperature is between a second lower range. Ifthe answer is yes, the condenser fan is turned off. On the other hand,if the temperature is below even that second range, the condenser fan isturned off, and the SMV is moved to a predetermined open amount.

This method is intended to provide a somewhat “fail safe” method ofensuring lubricant will be delivered back to the compressor, even if thedischarge pressure transducer is no longer functioning.

Although preferred embodiments of this invention have been disclosed, aworkers of ordinary skill in the art would recognize that variousmodifications of this method would be within the scope of thisinvention. For that reason, the following claims should be studied todetermine the true scope and content of this invention.

1. A method of monitoring operation of a refrigerant cycle comprisingthe steps of: (a) monitoring for a condition indicative of an inadequateflow of lubricant; and (b) taking preventative actions should saidmonitoring of step (a) indicate that an inadequate flow of lubricantexists, said preventative action providing lubricant flow.
 2. The methodas set for in claim 1, wherein said monitoring includes the step ofmonitoring a discharge pressure downstream of a compressor.
 3. Themethod as set forth in claim 1, wherein the preventative steps includingturning off a condenser fan associated with a condenser heat exchanger.4. The method as set forth in claim 3, wherein once the condenser fanhas been turned off, the condition continues to be monitored, and thecondenser fan is maintained off until the condition has changed toindicate that the refrigerant cycle is moving to a condition where it isless likely that there will be an inadequate flow of lubricant. 5.(canceled)
 6. The method as set forth in claim 5, wherein once theheater has been turned on, the condition continues to be monitored, andthe heater maintained on until the condition is changed to indicate thatthe refrigerant cycle is moving to a condition where it is less likelythat there will be an inadequate flow of lubricant. 7-10. (canceled) 11.The method as set forth in claim 1, wherein three or less preventativeactions are taken in serial order, with said preventative actionsincluding turning off a condenser fan, turning on a heater associatedwith an evaporator, and controlling the opening in a suction modulationvalve.
 12. (canceled)
 13. The method as set forth in claim 11, whereinthe control of said suction modulation valve is taken as the firstcorrective action.
 14. A refrigerant cycle comprising: a compressordelivering a refrigerant to a condenser, said condenser having a fan forblowing air over said condenser, said refrigerant being delivered fromsaid condenser to a downstream expansion device, and said refrigerantbeing delivered from said expansion device to an evaporator, refrigerantbeing delivered from said evaporator through a modulating valve back tothe compressor; and a control for controlling said refrigerant cycle,said control monitoring a condition within said refrigerant cycle, anddetermining whether conditions are indicative of a an inadequate flow oflubricant, said control taking a corrective action to provide lubricantflow if said conditions are indicative of an inadequate flow oflubricant.
 15. The refrigerant cycle as set for in claim 14, whereinsaid monitoring includes the step of monitoring a discharge pressuredownstream of the compressor.
 16. The refrigerant cycle as set forth inclaim 15, wherein the preventative steps including turning off acondenser fan associated with a condenser heat exchanger.
 17. Therefrigerant cycle as set forth in claim 16, wherein once the condenserfan has been turned off, the condition continues to be monitored, andthe condenser fan is maintained off until the condition has changed toindicate that the refrigerant cycle is moving to a condition where it isless likely that there will be an inadequate flow of lubricant. 18-26.(canceled)
 27. The method as set forth in claim 1, wherein a compressorassociated with the refrigerant cycle continues to be driven while thestep (b) occurs.
 28. The refrigerant cycle as set forth in claim 14,wherein said compressor continues to be driven while the correctiveaction is taking place.